The present invention relates to a method for requesting grant for MAC protocol in PON and a computer readable storage medium storing instructions for executing a method for requesting a grant in an optical subscriber network in which an Optical Line Termination (OLT) connects remote Optical Network Units (ONUs) by an optical fiber and passive optical devices.
In an Asynchronous Transfer Modexe2x80x94Passive Optical Network (ATM-PON) being considered as one of the representative forms of PONs, an OLT broadcasts data to ONUs through an optical fiber on which a splitter splits a string of downstream data into maximum sixty-four downstream data strings. On the other hand, upstream data strings from the ONUs is multiplexed by a combiner to be transmitted through the optical fiber to the OLT. The maximum distance between an OLT and an ONU can be extended up to 20 km. ATM-PON can be deployed in the forms such as Fiber-to-the-Cabinet (FTTCab), Fiber-to-the-Home (FTTC), Fiber-to-the-Building (FTTB), and Fiber-to-the-Home (FTTH), according to the place in which an ONU is installed and the processing capacity of an ONU. Especially, an ONU, such as FTTH which is installed in a home, is called an Optical Network Termination (ONT). In addition, ATM-PON supports 155.52 Mbps or 622.04 Mbps as a downstream transmission bit rate, and 155.52 Mbps as an upstream transmission bit rate. Furthermore, variable transmission bit rates for both downstream and upstream data can be allocated to respective ONUs on a permit basis by an OLT.
As important techniques in ATM-PON, there are xe2x80x9crangingxe2x80x9d and MAC protocols. Downstream data in ATM-PON can be transmitted without special difficulties except to a security problem, because the downstream data does not suffer cell collisions that can be occurred by different time delays due to different distances between an ONU and respective ONUs. However, upstream data transmission from ONUs is not free from the cell collision problem, because upstream data strings, arrived at a combiner, from different distances would contend to simultaneously occupy a certain location in a cell frame. Even though it""s desirable for ATM-PON to have means to solve the problem, passive devices such as a splitter and a combiner do not have the means.
Therefore, xe2x80x9crangingxe2x80x9d protocol has been devised to solve the problem, and the protocol was selected as an international standard. In the xe2x80x9crangingxe2x80x9d protocol, an OLT senses a newly registered ONU; then, measures the distance from the ONU; calculates an appropriate equalized delay for the ONU; and allocates the equalized delay to the ONU. Because of the time delay adjustment by the OLT, all ONUs connected with the OLT can be considered as located at the same distance.
However, MAC protocol in ATM-PON, which dynamically allocates appropriate bit rates to respective ONUs, has not been standardized yet. The MAC protocol comprises Grant Request Protocol (GRP) and Grant Distribution Algorithm (GDA). The GRP is a process and procedure in which an ONU transmits its information to an OLT in order to receive time slots for its upstream cell (data) transmission, and in which the OLT gives a grant for cell (data) transmission with appropriate time slots. The GDA is an algorithm which distributes grants by an appropriate method based on information from ONUs, before the OLT gives the grant for cell transmission.
Referring to FIG. 3, an ATM-PON upstream frame comprises successive fifty-three time slots, and each time slot comprises fifty-six bytes that includes a three byte overhead and a fifty-three byte cell. The three byte overhead is used to maintain a specific span between two successive cells, and to synchronize bits or bytes.
On the other hand, an ATM-PON downstream frame, which is successively repeated, in accordance with the International Standard comprises successive fifty-six time slots, and each time slot has fifty-three bytes which is identical with an ATM cell.
In an ATM-PON downstream frame, the first and twenty-ninth time slots among fifty-six time slots are periodically occupied by Physical Layer Operation and Maintenance (PLOAM) cells. A PLOAM cell header is generally identical with a Physical Layer cell, but a Payload Identifier is coded to 110 which identifies the PLOAM cells as an Operation and Maintenance (OAM) cell for ATM-PON.
Referring FIG. 4, the forty-eight byte payload structure of the PLOAM cell will be explained as in the following.
First, the xe2x80x9cIDENTxe2x80x9d field specifies whether the present location of the PLOAM cell in an ATM-PON downstream frame is in the first time slot or the twenty-ninth time slot.
Second, the xe2x80x9cSYNC1xe2x80x9d and xe2x80x9cSYNC2xe2x80x9d fields that follow the xe2x80x9cIDENTxe2x80x9d field are for providing a standard clock of 1 KHz.
Third, the other fields are roughly divided to both grant fields and message fields. The grant fields have information about who can use a specific time slot. In greater detail, GRANT k (1xe2x89xa6kxe2x89xa627) of the j th (j=1,2) PLOAM cell means that an entity (which may not be an ONU) to which is allocated GRANT K as an entity""s ID is now using the 27(jxe2x88x921)+k th time slot among fifty-three upstream time slots.
The last grant (GRANT 27) of the second PLOAM cell, namely in case of xe2x80x9cj=2, k=27,xe2x80x9d is allocated nothing, because an upstream frame comprises only fifty-three time slots. Upstream transmission grants given from an OLT to an ONU in the International Standard are xe2x80x9cranging grant,xe2x80x9d xe2x80x9cunassigned grant,xe2x80x9d and xe2x80x9cidle grant,xe2x80x9d and each grant is respectively defined as xe2x80x9c11111101,xe2x80x9d xe2x80x9c11111110,xe2x80x9d and xe2x80x9c11111111.xe2x80x9d The remainders of the grants can be assigned by an OLT. In the procedure for xe2x80x9cranging,xe2x80x9d such assignments can be performed through a message field of a PLOAM cell, and the message field is filled up with parameters and commands from an OLT to an ONU. Time slots in an upstream frame distinct from a downstream frame can be filled with user cells. Upstream PLOAM cells, and a divided slot, depending on grants that an OLT gives. Among the three kinds of time slots, a divided time slot occupies a fifty-six byte upstream time slot in which one or more of mini slots are filled out in order to efficiently use the divided time slot. Requests for grants from ONUs or other entities are filled in the mini time slots.
Therefore, the size of a mini slot can be arbitrarily fixed between one byte and fifty-three bytes. However, in fact, the undefined size and period of the mini slots cause confusion in embodiment of equipment, and put an obstacle in standardization for the equipment.
An object of the present invention is to provide a data-storing device that can be read by a computer, and that stores a method for requesting a grant in an optical subscriber network in which an Optical Line Termination (OLT) connects remote Optical Network Units (ONUs) by an optical fiber and passive optical devices.
In accordance with an aspect of the present invention, there is provided a method for requesting a grant for Medium Access Control (MAC) being applied to a Passive Optical Network (PON) system, the method comprising the steps of: a) deciding a period of a mini slot (Tms), over which a plurality of Optical Network Units (ONU) transmit upstream cells to an Optical Line Termination (OLT); b) deciding a period of a divided slot (Tds) and a link overhead (Co) based on the period of the mini slot (Tms); c) calculating a length of the mini slot based on information to be transmitted to the OLT and a protocol being used; and d) requesting a grant for a MAC protocol between the plurality of the ONUs by calculating and allocating optimal parameters based on the length of the mini slot.
In accordance with another aspect of the present invention, there is provided a computer readable storage medium storing instructions for executing a method for requesting a grant for Medium Access Control (MAC) in a Passive Optical Network (PON) system having one or more processors, the method comprising the steps of: a) deciding a period of a mini slot (Tms), which is necessary for a plurality of Optical Network Units (ONU) to transmit upstream cells to an Optical Line Termination (OLT); b) deciding a period of a divided slot (Tds) and a link overhead (Co) based on the period of the mini slot (Tms); c) calculating a length of the mini slot based on information to be transmitted to the OLT and a protocol being used; and d) requesting a grant for a MAC protocol between the plurality of the ONUs by calculating and allocating optimal parameters based on the length of the mini slot.